1. Technical Field
This application relates to computer storage devices, and more particularly to the field of providing copies of portions of data stored on a computer storage device.
2. Description of Related Art
Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units (host adapters), disk drives, and disk interface units (disk adapters). Such storage devices are provided, for example, by EMC Corporation of Hopkinton, Mass. and disclosed in U.S. Pat. No. 5,206,939 to Yanai et al., 5,778,394 to Galtzur et al., U.S. Pat. No. 5,845,147 to Vishlitzky et al., and U.S. Pat. No. 5,857,208 to Ofek. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels of the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical volumes. The logical volumes may or may nor correspond to the actual disk drives.
In some instances, it may desirable to provide a copy of a logical volume where the copy is then accessed by other processes. For example, to test new software on actual stored data, a copy of a logical volume containing the data may be made and the copy, as opposed to the original data, may be used to test new software. Once the test is complete, the copy may be eliminated. Thus, the new software is tested on actual data without affecting the actual data. This reduces the likelihood that testing new software and/or functionality will corrupt actual data.
One difficulty with making such copies is that they require as much storage space as the logical volume from which the data is obtained since the copy process simply creates a new volume containing all the data of the original volume. In addition, in some instances, the differences between the original volume and the copy are minimal. Thus, the extra storage space required for such a copy of a logical volume is used somewhat inefficiently since it merely duplicates already-existing data. Accordingly, it would be desirable to provide a mechanism for copying data in a way that uses storage space efficiently.